A wind turbine comprises a rotor with a hub and rotor blades. During the operation of the wind turbine, the wind interacts with the rotor blades and rotates the rotor. The rotation of the rotor is transferred to an electric generator to generate electric power. In a direct driven wind turbine, the rotor and the generator are directly coupled, without a gearbox between the rotor and the generator. Thus, the rotor of the wind turbine is directly coupled to the rotor of the electric generator.
The rotor of the wind turbine and the rotor of the electric generator belong to the rotational part of the wind turbine. The rotational part of the wind turbine is supported by a bearing. The bearing comprises a stationary ring and a rotatable ring. The stationary ring is attached to the stationary part of the wind turbine, and the rotatable ring is connected to the rotatable part of the wind turbine. The bearing is often called the main bearing of the wind turbine. The main bearing is a rolling element bearing or a sliding bearing.
The lifetime of the bearing, and the width of the air gap in the generator, depend on the clearance in the bearing. Thus it is important to control the clearance in the bearing.
During the start-up of the wind turbine and during operation, the bearing warms up due to friction in the bearing. Due to the warming, the parts of the bearing expand. This expansion influences the clearing of the bearing. The clearance stays mainly the same, as long as the stationary ring and the rotatable ring of the bearing expand mainly equally. This is the case when they show mainly the same temperature.
During the start-up of the wind turbine or changes in the load in the rotating part of the wind turbine, the temperature in the bearing changes. Due to the different mass of the rotatable ring and the stationary ring of the bearing, the rings might warm up or cool down in a different time. This leads to a difference in the temperature of the bearing rings. Thus, the clearance of the bearing is changing. A changing clearance has a negative effect on the lifetime of the bearing and/or the width of the air gap of the generator.
It is therefore known to cool the bearing with an integrated oil lubrication system, where the oil is cooled and is pumped through the bearing. This shows the problem that the exchange rate of the oil in the bearing is too low to cool the whole bearing with the lubrication oil. WO 2011082836 A1 describes a cooling system of a bearing of a direct driven wind turbine, whereby the cooling system comprises at least one heat sink being in thermal contact to the inner ring of the bearing and a heat dissipater being in thermal communication with the heat sink. The system shows the disadvantage that a cooling liquid is circulating though the heat sink at the bearing and the heat dissipater. Thus, the installation of ducts for the cooling fluid is necessary that guide the cooling liquid, and there is a certain risk that the liquid might leak from the system.
Accordingly, a need exists for an improved arrangement to cool the bearing.